The present invention relates to the fields of blades acting on fluids, particularly for propulsion of craft, but possibly also for blades acting on fluids in pumps.
Particularly the invention relates to blades acting on water for the propulsion of water craft; in rotodynamic machines such as propellers on in-board, outboard, or stern drive units on boats such as pleasure craft, screws on larger boats and ships, impellors in jet drive units. It is also possible that the invention may have application in propellers for displacing air, such as in aeroplanes, hovercraft, and rotors in helicopters.
In addition, the invention might have application impellers in pumps, and turbines and the like.
A difficulty with propellers in water craft is that as the speed of the propeller increases, there is a loss of efficiency. Much of this loss is induced by the rotary motion of the blades of the propeller imparting a rotary motion in the water, and also giving rise to turbulence, eddies in flow and slippage. As the speed further increases, an even more catastrophic effect known as cavitation can be observed.
The invention seeks to ameliorate the aforementioned problems.
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In accordance with one aspect of the invention there is provided in a blade in a rotodynamic machine for acting on a fluid, the blade having two surfaces, one on either side thereof, at least one surface of which acts on said fluid; a plurality of apertures extending through said blade between said two surfaces in a direction substantially normal to the radial extent of said rotodynamic machine; said plurality of apertures located in positions spread substantially evenly throughout said blade.
Preferably said apertures have a cross-sectional area of up to 50% of the entire blade area.
Preferably said apertures have a cross-sectional area of up to 20% of the entire blade area.
Preferably said apertures have a cross-sectional area of up to 10% of the entire blade area.
Preferably said apertures have a cross-sectional area of up to 5% of the entire blade area.
Preferably said apertures have a cross-sectional area of between 1% and 3% of the entire blade area.
Preferably said apertures have a cross-sectional area of about 2% of the entire blade area.
Preferably the apertures have a diametric aspect ratio of up to 1:4. The apertures may be rectangular or elliptical, with such a diametric aspect ratio.
Preferably the apertures have a diametric aspect ratio of up to 1:2.
Preferably the apertures are circular in cross-section (diametric aspect ratio of 1:1).
Preferably the apertures include a bevelled leading edge on the front of the blade.
The size of the apertures is dependent upon factors such as the speed of the blade through the fluid. In this regard, an aperture size of 2.5 to 3.5 mm would be appropriate where the blade is a blade in a propeller for use on a power boat. Faster rotational speeds or a finer pitch may require larger apertures. In addition, where the blade is a blade in a propeller, where there is a finer pitch or faster rotational speeds are employed, the apertures may comprise a larger cross sectional area of the blade.
In the case of a propeller it is preferred that the size of the apertures at the outer edge (where the linear speed is faster) is larger than the size of the apertures nearer the hub. It is preferred that the size of the apertures vary progressively or in stepwise manner, decreasing from the outer edge of the propeller toward the hub. For a propeller in power boat or on an outboard motor, the size of the aperture near the outer edge of the blade may be in the order of 2.8 mm to 3.0 mm, while the size of the apertures closest to the hub may be around 2.0 mm to 2.2 mm. The size of the apertures from the outer edge of the blade, toward those located closest to the hub, progressively decreases. It is most preferred that the size of the apertures between the outer edge of the blade and toward the hub is selected so that the flow rate of water flowing through each aperture is substantially constant, across the blade, so that the effect imparted is even across the entire propeller.
Preferably said plurality of apertures are aligned with their axial extent extending up to 30xc2x0 from the direction of travel of the blade relative to the axis of the propeller.
Preferably said plurality of apertures are aligned with their axial extent extending up to 20xc2x0 from the direction of travel of the blade relative to the axis of the propeller.
Preferably said plurality of apertures are aligned with their axial extent extending up to 10xc2x0 from the direction of travel of the blade relative to the axis of the propeller.
Preferably said plurality of apertures are aligned with their axial extent extending up to 5xc2x0 from the direction of travel of the blade relative to the axis of the propeller.
Preferably said plurality of apertures are aligned with their axial extent extending substantially in the direction of travel of the blade relative to the axis of the propeller.
The angle referred to above is the angle relative to the direction of rotational travel relative to the axis of the propeller, and not including any component derived from propulsion imparted by the blade. In the case of finer pitched blades, it will be necessary to have an angle of the aperture greater than about 20xc2x0. The finer the pitch of a propeller, the greater the angle of inclination of the apertures.
In accordance with a second aspect of the invention there is provided a rotodynamic machine having at least one blade as hereinbefore described.
In order to balance the rotodynamic machine, it is preferred that there be two or more of said blades. In practice there will be a plurality of blades in a dynamically balanced configuration, usually comprising three or more blades.
It will be understood that the rotodynamic machine may be a propeller on an inboard, outboard, or stern drive unit for a boat such as a pleasure craft, a propeller or screw on a ship, or an impellor in a jet drive unit in a jet boat. Similarly, the rotodynamic machine may be an impellor in a pump, a turbine in a hydro-electric power generation plant. It will also be understood that the rotodynamic machine may be a propeller employed on an aircraft or a rotor on a helicopter.